Basic requirements of materials for use in pressurised gas containment systems include: providing adequate fabricability to allow manufacture of the system and the capability to provide adequate strength, ductility, toughness, corrosion resistance, and resistance to all forms of time-dependence degradation of mechanical properties in the final product.
In the past, these requirements have restricted the use of aluminium alloys in commercial gas cylinders to those with peak strengths below about 450 MPa. An ill-fated attempt to exceed this strength level was made in the early 1970s, when a 7000 series aluminium alloy gas cylinder was introduced into the marketplace and resulted in the recall of all cylinders due to severe stress corrosion cracking initiating after limited service life that eventually would have led to catastrophic failures.
U.S. Pat. No. 4,439,246 (Gerzat) describes a method of making pressurised gas cylinders from 7475 alloy. A billet of the alloy was homogenised for 12 hours at 465.degree. C.; hot (or alternatively cold) extruded; necked; solution annealed and quenched; and finally aged by the two step tempering type T73 treatment.
European Patent specification 257 167 (Gerzat) reports that the products (of the aforesaid U.S. patent) were found to be unsuitable after extensive testing, despite their very high level of fracture toughness, their good mechanical strength and excellent stress corrosion resistance in the T73 condition. The problem is solved, according to the European patent specification, by use of an alloy comprising 6.25-8.0% Zn; 1.2-2.2% Mg; 1.7-2.8% Cu; 0.15-0.28% Cr; and Fe+Si preferably &lt;0.25%. As-cast billets of this composition are subjected to hot backward extrusion; drawing; necking; solution heat treating and quenching; and precipitation heat treating to a variety of over-aged conditions.
There is a need for pressurised gas cylinders with a higher strength to weight ratio, and in which any failure is preferably confined to the cylindrical part and does not spread to or occur at either the base or the shoulder.